LT1630/LT1631
16
16301fa
applicaTions inForMaTion
Input Offset Voltage
The offset voltage changes depending upon which input
stage is active, and the maximum offset voltages are
trimmed to less than 525µV. To maintain the precision
characteristics of the amplifier, the change of V
OS
over the
entire input common mode range (CMRR) is guaranteed
to be less than 525µV on a single 5V supply.
Input Bias Current
The input bias current polarity depends on the input
common mode voltage. When the PNP differential pair is
active, the input bias currents flow out of the input pins.
They flow in the opposite direction when the NPN input
stage is active. The offset voltage error due to input bias
currents can be minimized by equalizing the noninverting
and inverting input source impedance.
Output
The outputs of the LT1630/LT1631 can deliver large load
currents; the short-circuit current limit is 70mA. Take care to
keep the junction temperature of the IC below the absolute
maximum rating of 150°C (refer to the Power Dissipation
section). The output of these amplifiers have reverse-biased
diodes to each supply. If the output is forced beyond either
supply, unlimited current will flow through these diodes.
If the current is transient and limited to several hundred
mA, no damage to the part will occur.
Overdrive Protection
To prevent the output from reversing polarity when the
input voltage exceeds the power supplies, two pairs of
crossing diodes D1 to D4 are employed. When the input
voltage exceeds either power supply by approximately
700mV, D1/D2 or D3/D4 will turn on, forcing the output
to the proper polarity. For this phase reversal protection
to work properly, the input current must be limited to less
than 5mA. If the amplifier is to be severely overdriven,
an external resistor should be used to limit the overdrive
current.
The LT1630/LT1631’s input stages are protected against
large differential input voltages by a pair of back-to-back
diodes D5/D6. When a differential voltage of more than
0.7V is applied to the inputs, these diodes will turn on,
preventing the emitter-base breakdown of the input transis-
tors. The current in D5/D6 should be limited to less than
10mA. Internal 225Ω resistors R6 and R7 will limit the
input current for differential input signals of 4.5V or less.
For larger input levels, a resistor in series with either or
both inputs should be used to limit the current. Worst-case
differential input voltage usually occurs when the output is
shorted to ground. In addition, the amplifier is protected
against ESD strikes up to 3kV on all pins.
Capacitive Load
The LT1630/LT1631 are wideband amplifiers that can
drive capacitive loads up to 200pF on ±15V supplies in a
unity-gain configuration. On a 3V supply, the capacitive
load should be kept to less than 100pF. When there is a
need to drive larger capacitive loads, a resistor of 20Ω
to 50Ω should be connected between the output and the
capacitive load. The feedback should still be taken from
the output so that the resistor isolates the capacitive load
to ensure stability.
Feedback Components
The low input bias currents of the LT1630/LT1631 make it
possible to use the high value feedback resistors to set the
gain. However, care must be taken to ensure that the pole
formed by the feedback resistors and the total capacitance at
the inverting input does not degrade stability. For instance,
the
LT1630/LT1631 in a noninverting gain of 2, set with
two 20k resistors, will probably oscillate with 10pF total
input capacitance (5pF input capacitance and 5pF board
capacitance). The amplifier has a 5MHz crossing frequency
and a 52° phase margin at 6dB of gain. The feedback
resistors and the total input capacitance form a pole at
1.6MHz that induces a phase shift of 72° at 5MHz! The
solution is simple: either lower the value of the resistors
or add a feedback capacitor of 10pF or more.
